Rats

In Poul Anderson's "In Memoriam," in the long post-human future of Earth, rats variously evolve into grass-eaters, predators, winged raptors and flippered water-dwellers. Some climb into trees, develop hands, return to the ground and grow bigger bodies and brains but never use fire or complex tools.

I used to think that human beings evolved because some quadrupeds evaded predators by climbing into trees where they were safe and free to chatter and grasped branches, thus developing opposable thumbs. Then, their descendants descended to the plains walking upright in groups with forelimbs freed to manipulate their environment. This account at least had the merit of plausibility.

The Time Patrol Academy is:

"...in the Oligocene period, a warm age of forests and grass-lands when man's ratty ancestors scuttled away from the tread of giant mammals." (Time Patrol, pp. 5-6)

Those "...ratty ancestors..." would have been the pre-arboreal quadrupeds. Bryan Talbot's The Tale Of One Bad Rat (see here) also states that human beings are descended from something like rats. And this may be the case. However, I am now told that what evidence there is indicates that bipedalism preceded opposable thumbs.